This invention relates to a distillation column with vaporous sidestream removal and a condenser for condensing the vaporous product removed.
The bottom product removed in liquid form from the bottom of distillation columns often contains high-boiling organic residues or salts. To remove these unwanted secondary products, the main product is removed in vaporous form in a sidestream. Where a large separation factor is present between both, a sidestream removal which consists solely of a sidestream condenser and which represents a distillation stage is sufficient for this purpose. If this separation is inadequate, a sidestream column has to be arranged between the main distillation column and the side-stream condenser.
It is now standard practice to connect a descending sidestream condenser to the vapor pipe for the vaporous sidestream removal. The condensate is removed by a pump of which the output is either constant or is regulated via a cascade by the temperature, the inflow volume or the bottom level of the column. The pump is operated in such a way that the lower part of the cooling surface of the sidestream condenser is covered with condensate. If less liquid condenses than is removed by the pump, the filling level in the sidestream condenser falls so that more cooling surface is uncovered and the accumulation of condensate increases again. By contrast, if more liquid condenses than is removed by the pump, the liquid level in the condenser increases, the cooling surface becomes covered and the accumulation of condensate falls. Automatic regulation is thus established.
This method of sidestream removal presents problems when the sidestream condenser is filled with air when the installation is started up or when, during distillation, the vapors contain non-condensable gases. These "inerts" (air or non-condensable components) can only be removed to a limited extent by the discharge pump. The path through the sidestream pipe (vapor pipe) back to the column against the flow of vapor is also blocked. The inerts thus cover the cooling surfaces of the sidestream condenser, impair heat transfer and gradually lead to its failure. The situation can only be remedied by temporarily filling the condenser with air, the condenser then having to be separated from the column to avoid product losses. The operating times of the installation are thus shortened.